SWIG: Handle negative offsets in VSIFSeekL

Author: dbaston

autotest/gcore/vsifile.py

@@ -1415,17 +1415,27 @@ def test_vsifile_class_read_ascii(tmp_path):
     assert f.read(10) == "permission"
 
     # skip a character
-    f.seek(1, os.SEEK_CUR)
+    assert f.seek(1, os.SEEK_CUR) == 0
     assert f.read(9) == "is hereby"
 
-    f.seek(0, os.SEEK_SET)
+    # seek backwards
+    assert f.seek(-2, os.SEEK_CUR) == 0
+    assert f.read(2) == "by"
+
+    # jump to beginning
+    assert f.seek(0, os.SEEK_SET) == 0
     assert f.read(10) == "permission"
 
+    # can't jump before the beginning
+    pos = f.tell()
+    assert f.seek(-2, os.SEEK_SET) == -1
+    assert pos == f.tell()
+
     # jump to end
-    f.seek(0, os.SEEK_END)
+    assert f.seek(0, os.SEEK_END) == 0
     assert f.read(10) == ""
 
-    f.seek(-7, os.SEEK_END)
+    assert f.seek(-7, os.SEEK_END) == 0
     assert f.read() == "person\n"
 
     f.close()

autotest/pymod/gdaltest.py

@@ -2185,7 +2185,7 @@ def write(self, x):
         gdal.VSIFWriteL(x, 1, len(x), self._fp)
 
     def seek(self, offset, whence=0):
-        gdal.VSIFSeekL(self._fp, offset, whence)
+        return gdal.VSIFSeekL(self._fp, offset, whence)
 
     def tell(self):
         return gdal.VSIFTellL(self._fp)

swig/include/cpl.i

@@ -906,8 +906,33 @@ int VSIFFlushL( VSILFILE* fp );
 
 VSI_RETVAL VSIFCloseL( VSILFILE* fp );
 
+// Wrap VSIFSeekL to allow negative offsets
+%rename (VSIFSeekL) wrapper_VSIFSeekL;
+%inline {
+#if defined(SWIGPYTHON)
+int wrapper_VSIFSeekL( VSILFILE* fp, GIntBig offset, int whence) {
+#else
+VSI_RETVAL VSIFSeekL( VSILFILE* fp, long offset, int whence) {
+#endif
+if (offset < 0) {
+    switch (whence) {
+        case SEEK_END: VSIFSeekL(fp, 0, SEEK_END);
+        case SEEK_CUR:
+            offset = VSIFTellL(fp) + offset;
+            whence = SEEK_SET;
+            break;
+        default:
+            VSIError(VSIE_FileError, "Cannot use negative offset with SEEK_SET");
+            return -1;
+    }
+}
+
+return VSIFSeekL(fp, offset, whence);
+}
+}
+
+
 #if defined(SWIGPYTHON)
-int     VSIFSeekL( VSILFILE* fp, GIntBig offset, int whence);
 GIntBig    VSIFTellL( VSILFILE* fp );
 int     VSIFTruncateL( VSILFILE* fp, GIntBig length );
 
@@ -919,7 +944,6 @@ int     VSISupportsSparseFiles( const char* utf8_path );
 
 int     VSIFGetRangeStatusL( VSILFILE* fp, GIntBig offset, GIntBig length );
 #else
-VSI_RETVAL VSIFSeekL( VSILFILE* fp, long offset, int whence);
 long    VSIFTellL( VSILFILE* fp );
 VSI_RETVAL VSIFTruncateL( VSILFILE* fp, long length );
 #endif

swig/include/python/gdal_python.i

@@ -371,7 +371,7 @@ void wrapper_VSIGetMemFileBuffer(const char *utf8_path, GByte **out, vsi_l_offse
         raise ValueError("I/O operation on closed file.")
 %}
 
-%pythonprepend VSIFSeekL %{
+%pythonprepend wrapper_VSIFSeekL %{
     if args[0].this is None:
         raise ValueError("I/O operation on closed file.")
 %}